Microencapsulation techniques have been widely applied as one of means for enclosing (or sealing) various materials (or core materials) such as a dye, a perfume (or aromatic), a crystalline liquid, an enzyme, a catalyst, and an adhesive. The advantages of such techniques are in that the handling of these core materials can be improved and that the functions of the core materials can be maintained or retained for a long period of time.
On the other hand, display techniques are utilized in a broad range from a displaying method for displaying an image or character information to a visualizing method using a mode in such as a liquid crystal mode, a plasma emission mode, or an EL (electroluminescence) mode. In recent years, as various electronic apparatuses (or devices) are miniaturized due to rapid advance of semiconductor technology, so increase the demand for the miniaturization, weight-saving, lower driving voltage, less electricity to work, and thinner flat panel of display devices. As new display method for responding to these requirements, there are proposed electrophoretically image-displaying devices (or apparatuses) writable on the display surface, which comprises a dispersed system (core material) in which electrophoretic particles (or electrophoretically-movable particles) are dispersed in a disperse medium, microcapsules encapsulating the dispersed system, and being interposing between electrode plates for migrating or moving the electrophoretic particles in the microcapsules between the electrode plates by applying an electric field.
Japanese Patent Application Laid-Open No. 119264/1999 (JP-11-119264A) discloses a display device comprising a disperse system in which charged particles are dispersed into a disperse medium, a number of microcapsules encapsulating the disperse system, and a pair of opposed electrodes which are so disposed as to insert these microcapsules therebetween. In the display device, a display manner is conducted by changing the distribution condition of the charged particles depending on an action of a controlled voltage to change the optical reflexivity. The particle size of the charged particles is about 1/1000 to ⅕ relative to that of the microcapsules, and the dispersivity in the particle size distribution of the charged particles (volume-average particle size/number-average particle size) is 1 to 2. Japanese Patent Application Laid-Open No. 202372/1999 (JP-11-202372A) discloses a display device comprising at least two kinds of charged particles constituting the disperse system and is encapsulated in the microcapsule, and a disperse medium containing a surfactant, wherein the charged particles contain at least one member selected from titanium oxide and carbon black.
Japanese Patent No. 2551783 discloses an electrophoretic display device using microcapsules encapsulating a disperse system, as microcapsules disposed between the electrodes, in which the disperse system comprises a colored disperse medium, and at least one kind of an electrophoretic particle different in optical property from the colored disperse medium. Further, Japanese Patent Application Laid-Open No. 503873/2001 (JP-2001-503873A) discloses an electrophoretically displaying device comprising an arrangement of discrete microscopic containers (or microcapsules); first and second electrodes disposed on and covering opposite sides of the arrangement, at least one of the electrodes being substantially visually transparent; a means for creating a potential difference between the two electrodes; and within each container, a suspension comprising a dielectric fluid and particles exhibiting surface charges in the dielectric fluid, the dielectric fluid and the particles contracting visually, and the potential difference causing the particles to migrate toward one of the electrodes.
In such a microcapsule, the wall of the microcapsule should be dense or closely for encapsulating a liquid as a core material. As a process for producing a microcapsule, physicochemical processes and chemical processes have been known, and these processes are suitably selected and utilized depending on applications of the microcapsule. As the physicochemical processes, a coacervation method using gelatin is well known, and is explained in detail in “Microcapsules (new edition, 1987)” attributed by Kondo Tamotsu et al. (published by Sankyoshuppan Co., Ltd.). Although this method is applied in a wide field, use of gelatin being a natural product causes variation of quality as a membrane material and deteriorates water resistance of the capsule, as a result the application is limited. Moreover, it is impossible to inhibit the formation of a core material-free coacervate particle/drop (as the core material, e.g., a coloring agent dispersed in oil), or the by-production of a capsule enclosing a plurality of core materials. Further, the coacervation method itself is a method for forming a capsule wall and fails to control the particle size of the resulting particles, and the particle size distribution of the particles depends on the dispersiveness of the core material. Therefore, according to the coacervation method, it is difficult to obtain a microcapsule encapsulating a disperse system in which a coloring agent is dispersed in an oil phase, with controlling the particle size and the particle size distribution in high yield.
As the chemical processes, in addition to an in-situ polymerization method (phase-separation method) which comprises allowing a reaction to proceed from a continuous aqueous phase for forming a wall made of an amino resin or others around a core material, there is known an interfacial polymerization in which an aqueous phase comprising a reaction component is reacted with an oil phase comprising another reaction component by polymerization or condensation on the phase boundary for forming a polymeric wall of the microcapsule. In a production process of an encapsulated ink encapsulating a disperse system in which a coloring agent is dispersed in an oil phase, an in-situ polymerization method using an amino resin is particularly utilized [for example, Japanese Patent Publication No. 27452/1993 (JP-5-27452B), Japanese Patent Publication No. 51339/1993 (JP-5-51339B), Japanese Patent Publication No. 53538/1993 (JP-5-53538B), and Japanese Patent Publication No. 53539/1993 (JP-5-53539B)]. However, since a large number of capsule particles without the coloring agent are produced as by-products in the encapsulation process, a step for removing not only by-product particles but also an emulsifying and dispersing agent is essential. Further, as the same as in the coacervation method, the particle size distribution depends on the dispersiveness of the core material. Moreover, as the interfacial polymerization method, there is known a method for forming a wall of a capsule by polymerizing a polyhydric alcohol existing in a continuous aqueous phase and an isocyanate monomer existing in an oil phase of a core material on the boundary surface [for example, Japanese Patent Application Laid-Open No. 000362/1994 (JP-6-000362A), Japanese Patent Application Laid-Open No. 343852/1994 (JP-6-343852A), Japanese patent Publication No. 37975/1986 (JP-61-37975B), Japanese Patent No. 2797960, and Japanese Patent No. 3035726]. This method has advantages that the formation of particles without a core material is inhibited. However, in the above-mentioned method, unreacted monomers remain in the oil phase and the aqueous phase, and electrophoretic properties of the colored fine particles is deteriorated due to highly polar isocyanate monomers remaining in the oil phase in use as an encapsulated ink for an electrophoretic display devices (or apparatuses). Further, as the same as in the encapsulation method by coacervation or in-situ polymerization, the particle size distribution depends on the dispersiveness of the core material.
Therefore, new microencapsulation techniques have been required for an electrophoretic particle microcapsulating ink, by which production of microcapsule particles without a coloring agent can be inhibited and the particle size can be controllable.
Incidentally, Japanese Patent Application Laid-Open No. 66600/1993 (JP-5-66600A) discloses, as a powdery toner for visualizing an electrostatic latent image, an encapsulated toner encapsulating a coloring agent within an anionic self-water dispersible resin. This document describes a copolymer, as an anionic self-dispersible resin, having 20 to 500 mg equivalent of an acid group (such as carboxyl group) per 10 g of a solid resin. Moreover, the document also discloses a process for production of the toner, which comprises subjecting a mixed composition containing the anionic self-water dispersible resin and a coloring agent to a dispersing treatment, forming a capsulated particle in an aqueous medium by phase inversion emulsification of the mixed composition, and separating the produced capsule particle from the aqueous medium for dryness; and a process for producing a toner which comprises, after the phase inversion emulsification, hydrolyzing the neutralized acid group to form a free acid group. An organic solvent and water which are utilized for the phase inversion are removed from the formed encapsulated toner by drying, and the resultant toner is used for fixation on an object by heat-melting. Therefore, in the encapsulated toner, the coloring agent cannot be moved in the capsule. Moreover, crosslinking of the resin deteriorates the fixing property of the toner.